Hadoop学习十五：Hadoop-Hdfs DataXceiverServer源码概述

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一.DataXceiverServer类图

 

二.DataXceiverServer

1.  Server used for receiving/sending a block of data.This is created to listen for requests from clients or other DataNodes.  This small server does not use the Hadoop IPC mechanism.DataXceiverServer用于接收和发送block数据，它监听着client或者其它DataNode的请求。DataXceiverServer没有采用RPC机制。DataXceiverServer是流式机制，而RPC是命令式接口。
2. DataXceiverServer每接收到一个请求，就会创建一个DataXceiver来处理该请求。

   class DataXceiverServer implements Runnable, FSConstants {
     public static final Log LOG = DataNode.LOG;
     
     ServerSocket ss;	
     DataNode datanode;
     // Record all sockets opend for data transfer
     Map childSockets = Collections.synchronizedMap(new HashMap());
     
     static final int MAX_XCEIVER_COUNT = 256;		//默认值是256,每个node最多可以起多少个DataXceiver,如果太多的话可能会导致内存不足
   
     BlockBalanceThrottler balanceThrottler;
     
     DataXceiverServer(ServerSocket ss, Configuration conf, DataNode datanode) {
       this.ss = ss;
       this.datanode = datanode;
     }
   
     public void run() {
       while (datanode.shouldRun) {
           Socket s = ss.accept();	//接受client请求
           s.setTcpNoDelay(true);
           //实例化DataXceiver，需要Socket和DataNode
           new Daemon(datanode.threadGroup, new DataXceiver(s, datanode, this)).start();
       }
     }
   }

三.DataXceiver类图

1.  DataXceiver处理Client或DataNode的五种请求(DataTransferProtocol接口定义)。

     public static final byte OP_WRITE_BLOCK = (byte) 80;
     public static final byte OP_READ_BLOCK = (byte) 81;
     /**
      * @deprecated As of version 15, OP_READ_METADATA is no longer supported
      */
     @Deprecated public static final byte OP_READ_METADATA = (byte) 82;
     public static final byte OP_REPLACE_BLOCK = (byte) 83;
     public static final byte OP_COPY_BLOCK = (byte) 84;
     public static final byte OP_BLOCK_CHECKSUM = (byte) 85;

    

   class DataXceiver implements Runnable, FSConstants {
   
   	Socket s;
   	final String remoteAddress; // address of remote side
   	final String localAddress; // local address of this daemon
   	DataNode datanode;
   	DataXceiverServer dataXceiverServer;
   
   	public DataXceiver(Socket s, DataNode datanode,
   			DataXceiverServer dataXceiverServer) {
   
   		this.s = s;
   		this.datanode = datanode;
   		this.dataXceiverServer = dataXceiverServer;
   		dataXceiverServer.childSockets.put(s, s);
   		remoteAddress = s.getRemoteSocketAddress().toString();
   		localAddress = s.getLocalSocketAddress().toString();
   	}
   
   	public void run() {
   		DataInputStream in = null;
   		in = new DataInputStream(new BufferedInputStream(NetUtils.getInputStream(s), SMALL_BUFFER_SIZE));
   		short version = in.readShort();	if ( version != DataTransferProtocol.DATA_TRANSFER_VERSION ) {
           throw new IOException( "Version Mismatch" );
         }	//version
   		boolean local = s.getInetAddress().equals(s.getLocalAddress());	//本地请求
   		byte op = in.readByte();		//op
   		switch (op) {
   		//读数据块
   		case DataTransferProtocol.OP_READ_BLOCK:
   			readBlock(in);
   			datanode.myMetrics.addReadBlockOp(DataNode.now() - startTime);
   			if (local)
   				datanode.myMetrics.incrReadsFromLocalClient();
   			else
   				datanode.myMetrics.incrReadsFromRemoteClient();
   			break;
   		//写数据块
   		case DataTransferProtocol.OP_WRITE_BLOCK:
   			writeBlock(in);
   			datanode.myMetrics.addWriteBlockOp(DataNode.now() - startTime);
   			if (local)
   				datanode.myMetrics.incrWritesFromLocalClient();
   			else
   				datanode.myMetrics.incrWritesFromRemoteClient();
   			break;
   		//替换数据块
   		case DataTransferProtocol.OP_REPLACE_BLOCK: 
   			replaceBlock(in);
   			datanode.myMetrics
   					.addReplaceBlockOp(DataNode.now() - startTime);
   			break;
   		//拷贝数据块
   		case DataTransferProtocol.OP_COPY_BLOCK:
   			// for balancing purpose; send to a proxy source
   			copyBlock(in);
   			datanode.myMetrics.addCopyBlockOp(DataNode.now() - startTime);
   			break;
   		//读取数据块校验码
   		case DataTransferProtocol.OP_BLOCK_CHECKSUM: // get the checksum of
   														// a block
   			getBlockChecksum(in);
   			datanode.myMetrics.addBlockChecksumOp(DataNode.now()
   					- startTime);
   			break;
   		default:
   			throw new IOException("Unknown opcode " + op
   					+ " in data stream");
   		}
   		dataXceiverServer.childSockets.remove(s);
   		}
   	}